Oxidation-resistant ferrous alloy

ABSTRACT

A low alloy steel for use as a substrate for aluminum or aluminum alloy coatings, the steel containing from 0.01% to 0.13% carbon, from 0.5% to 3% chromium, from 0.8% to 3% aluminum, from 0.4% to 1.5% silicon, from 0.1% to 0.6% manganese, from 0.1% to 1% titanium and remainder substantially iron. The steel has good oxidation resistance at elevated temperature, good weldability and formability, thereby enhancing its utility for fabrication into a variety of wrought coated products.

United States Patent [1 1 Jasper [4 1 Sept. 30, 1975 [73] Assignee:Armco Steel Corporation,

Middletown, Ohio [22] Filed: May 15, 1974 [211 App]. N0.: 470,175

[52] US. Cl. 75/124; 75/126 D; 75/126 F [51] Int. Cl. C22C 38/06; C22C38/26;

C22C 38/28 [58] Field of Search 75/124, 126 D, 126 F [56] ReferencesCited UNITED STATES PATENTS 11/1956 Herzog 75/124 X 3/1958 l-lerzog75/124 X 10/1972 Caule et a1 148/315 Primary Examiner-C. LovellAssistant Examiner-Arthur J. Steiner Attorney, Agent, or Firm-Melville,Strasser, Foster & Hoffman 5 7 ABSTRACT A low alloy steel for use as asubstrate for aluminum or aluminurrf alloy coatings, the steelcontaining from 0.01% to 0.13% carbon, from 0.5% to 3% chromium, from0.8% to 3% aluminum, from 0.4% to 1.5% silicon, from 0.1% to 0.6%manganese, from 0.1% to 1% titanium and remainder substantially iron.The steel has good oxidation resistance at elevated temperature, goodweldability and formability, thereby enhancing its utility forfabrication into a variety of wrought coated products.

3 Claims, No Drawings OXIDATION-RESISTANT FERROUS ALLOY This is adivision of application Ser. No. 373,278, filed June 25, 1973.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a chromium-aluminumsilicon-titanium steel of low alloycontent'for use as a substrate for aluminum or aluminum alloy coatings,and wrought coated products thereof havinggood resistance to oxidationat elevated temperatures up to about 1,700F, and resistance againstattack by hydrocarbon combustion products at elevated temperature,together with high strength.

2. Description of the Prior Art Increasingly stringent requirements forantipollution controls on motor vehicles has created a need forrelatively low cost alloys which will be oxidation resistant at elevatedtemperatures, for use in automotive exhaust systems, such as catalyticconverters, mufflers, and like articles.

Aluminum coated carbon steel has proved to be not completelysatisfactory for some high temperature applications. The automotiveindustry has substituted stainless steels such as Armco 409 (containing0.05% carbon, 11% chromium, traces of aluminum, residual nickel, 0.5%titanium and remainder iron) and other stainless steels containing 1 1%or more chromium. The cost of such steel is high, thus making itundesirable for proposed use in automotive exhaust systems, such ascatalytic converters, and the like. Moreover, although this stainlesssteel has fair oxidation resistance at elevated temperature, and goodformability, it does not adequately withstand attack by molten salts andhydrocarbon combustion products at elevated temperature. The provisionof an aluminum coating on such a steel has been found to result in aproduct having the desired properties, but this solution obviously addseven greater cost.

U.S. Pat. No. 3,698,964, issued October 17, 1972, to E. J. Caule et a1,discloses an iron base alloy with good oxidation resistance attemperatures of about 700 to 800 C (about l,300 to 1,475 F). The alloyof this patent contains up to 2% carbon, 1 to 5% chromium, l to 4%aluminum, and/or 1 to 4% silicon, up to 1.5% manganese, up to 2% copper,up to 0.20% total of nickel, molybdenum, vanadium and otheralloyingelements. Preferably, a combination of 2% chromium and 3%aluminum, or 3% chromium and 2% silicon, are used, with manganesepreferably up to 0.2%, copper not more than 0.5%, carbon not more than1% and most preferably from 0.01 to 0.25%.

The high carbon content of the steel of the Caule et al patent resultsin a brittle structure having very limited cold workability, poorwelding characteristics, and poor mechanical properties generally. Theoptional presence of molybdenum and vanadium adversely affects oxidationscale resistance, as well as adding to the cost. The harmful effect ofmolybdenum on oxidation resistance is reported in Stainless And HeatResisting Steels," by Colombier and Hochmann, p. 330, St. Mar tinsPress, New York, N.Y. (1968). Moreover, copper, nickel, and otheraustenite stabilizers in amounts greater than typical residual contentsof about 0.2% each are undesirable since it can cause a phase change,with a consequent change in volume, on heating and cooling. This volumechange results in cyclic heating and cooling.

U18. Pat. No. 2,825,669, issued Mar. 4, 1958, to E. M. Herzog, disclosesa steel, and a heat treatment therefor wherein a micro-structure isproduced having resistance to stress-corrosion cracking in wet hydrogensulfide atmospheres. The steel of this patent contains from 0.08 to0.20% carbon, from 0.60 to 5.0% chromium, from 0.15 to 1.20% aluminum,from 0.30 to 1.20% manganese, from 0.10 to 0.50% silicon, up to 0.50%molybdenum, up to 1.0% vanadium, up to 1.0% titanium and remainder iron.A heat treatment at 740 to 780 C, a second heat treatment at about 970to l,080 C followed by a water-quench and a tempering treatment at about625 to 670 C, are stated to result in the desired micro-structure and anultimate tensile strength of atleast ksi. Oxidation resistance atelevated temperature is not contemplated in this patent, and thecomposition would not inherently produce a steel having this property.Moreover, the presence of molybdenum and vanadium is deleterious forreasons set forth above.

Other patents disclosing low alloy steels containing chromium, aluminum,and/or silicon in varying amounts include US. Pat. Nos. 3,431,101;2,835,570; and 2,770,563.

None of the above patents discloses a low alloy steel having acompletely ferritic structure within the contemplated operatingtemperature range, and exhibiting in combination good oxidationresistance at elevated temperature, good resistance against attack byhydrocarbon combustion products, good weldability and formability andrelatively high strength. Hence, there still exists a great need for alow-cost alloy having the above combination of properties forfabrication into coated welded and wrought products such as spaceheaters, automotive exhaust systems, e.g., catalytic converters andmufflers, and the like.

, SUMMARY The present invention provides a low alloy steel containingchromium, aluminum, silicon and titanium (preferablywith a total alloycontent of less than about 5%) which provides in sheet form a substratefor aluminum or aluminum alloy coatings, the coated sheet beingreadily-formable into wrought articles having good oxidation resistanceat elevated temperature, good resistance against attack by hydrocarboncombustion products at elevated temperature, and relatively highretained strength at elevated temperature. In its broad'compositionranges, the steel of the invention consists essentially of from about0.01% to about 0.13% carbon, from about 0.5% to about 3% chromium, fromabout 0.8% to about 3% aluminum, from about 0.4% to about 1.5% silicon,from about 0.1% to about 0.6% manganese, from about 0.1% to about 1%titanium, and remainder iron except for incidental impurities.Molybdenum and vanadium are restricted to a maximum of about 0.05% each,and copper, nickel and other austenite stabilizers to less than about0.2% each. i

The carbon, chromium, aluminum, silicon and titanium percentage rangesare critical and departure therefrom results in loss of one or more ofthe above properties. Control of the critically low molybdenum,vanadium, copper, nickel and other austenite stabilizer contents is alsoessential.

spalling after Carbon is essential in an amount of at least about 0.01%in order to provide the necessary strength in the steel. More than about0.13% carbon cannot be tolerated because of its adverse effect upon theweldability, formability and and general mechanical properties of thesteel, and because it is a strong austenite former.

At least 0.5% chromium in combination with at least about 0.8% aluminumand about 0.4% silicon is necessary in order to provide good oxidationresistance. A maximum of 3% chromium should be observed in order tominimize cost and avoid processing difficulties.

- At least about 0.8% aluminum is necessary not only for oxidationresistance at elevated temperature but also to provide adequate tensilestrength. More than 3% aluminum results in a loss of formability andworkability.

At least about 0.4% silicon is essential since it cooperates with thechromium and aluminum to impart oxidation resistance. However, a maximumof about 1.5% silicon should be observed since amounts in excess thereofalso result in loss of formability and workability.

Titanium is essential in an amountof at least about 0.1% in order toimpart good weldability to the steel. Moreover, excess titanium overthat needed to stabilize carbon has been found to improve the oxidationresistance at elevated temperature. This excess can be slight in view ofthe high cost of titanium and of the relatively low residual sulfur,nitrogen and oxygen contents of the steel of the invention. Preferablythe titanium content is 8 times the carbon content, and a maximum ofabout 1% titanium should thus be observed at the carbon levelscontemplated herein. Since it is known that columbium and/or zirconiumgenerally function in an equivalent manner in stainless steels, it isconsidered within the scope of the invention to substitute columbiumand/or zirconium in-whole or in part for titanium. Such substitutionwould be on a stoichiometric basis, with a minimum weight ratio ofcolumbium or zirconium to carbon of 8:1, preferably at least about 10:1.Columbium and/or zirconium would thus range from about 0.10% to about1.5% if substituted for titanium.

Impurities at residual levels normal for ferritic stainless steels canbe tolerated in the steel of the invention. More specifically, a maximumof about 0.03% sulfur and a maximum of about 0.04% phosphorus do notadversely affect the properties of the steel. Molybdenum and vanadiumare undesirable in the steel of the invention as explained above, andare maintained at the minimum practicable levels. Copper and nickel aremaintained at a maximum of less than 0.2% each for reasons set forthabove.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the desirable novelcombination of properties is achieved in a steel having the broadcomposition ranges hereinabove set forth, optimum properties areobtained in a steel having the following preferred analysis by weightpercent:

Carbon about 0.04 to about 0.06% Chromium about 1.7 to about 2.1Aluminum about 1.7 to about 2.0

Silicon about 0.6 to about 0.9 7: Manganese about 0.2 to about 0.4 70Titanium about 0.1 to about 0.6

the titanium being about 8 times the carbon content and remainder ironexcept for incidental impurities.

In order to investigate the effect of the relative proportions ofchromium, aluminum, silicon and titanium onthe properties of the steel,a series of experimental heats was prepared and tested. .For purposes ofcomparison, tests were also conducted on plain carbon steel coated withaluminum and aluminum alloys containing up to 10% silicon, and on ArmcoType 409 stainless steel. The compositions of the experimental heats areset forth in Table 1 below.

Steels of the invention also containing 0.1% Cu, 0.04% Mo, and 0.03% V.

The above materials were hot rolled from 2100 F (ll49 C) from 1 inch by3 inch ingots to 0.1 inch thickness. Samples were annealed at 1,700 F(927 C) for 10 minutes, descaled and cold rolled to 0.05 inch thickness.It should be recognized that annealing the hot rolled material isoptional. Tensile strengths were determined on the cold rolled samplesat this stage while the remainder of the cold rolled strip was an-'nealed at 1600 F (871 C) for 6 minutes and pickled. This material wastested for the remaining mechanical properties reported below in TableI].

A consideration of the mechanical properties reported in Table 11indicates that the steels of the invention have ultimate tensilestrengths equivalent to those of comparable prior art alloys but haveimproved elongation. This is believed to result from the relatively lowcarbon contents (ranging from about 0.03% to about 0.07%). The yieldstrength and tensile strengths of the alloys containing about 1%aluminum were about 5 ksi and 4 ksi, respectively, less than thosecontaining 2% aluminum. Of greater significance is the comparison withthe 1% chromium alloy (Sample Code 15) also having low aluminum andsilicon contents. It will be noted that the yield strength of the 1%chromium alloy was about 12 ksi lower and the tensile strength about 8k'si lower than the steels of the invention containing from about 1.7%to about 2% aluminum with chromium ranging from 0.5% to 2%. At the sametime the elongation values of these steels of the invention were aboutequivalent to that of Sample Code 15. For an optimum combination ofmechanical properties, it is thus apparent that the carbon contentshould not exceed TABLE II 0.2% Olsen Sample YS UTS 71 Elong. HardnessCup Test Code (ksi) (ksi) in 2" (Rockwell B) Heightlnches Steels of thepresent invention Samples of the steels of Table l in the cold rolled,an nealed and pickled condition were surface ground, and a fullpenetration autogenous GTA weld was run down the longitudinal axis ofthe strip of each sample. 180 bend and Olsen cup test specimens were cutfrom the samples and tested with both root and face side in tension.These tests are reported in Table III.

From the data in Table 111 it is evident that optimum as-weldedductility is exhibited with about 1% aluminum and chromium in excess of1 At the 2% aluminum level better results apparently are obtained withchromium at about 2%.

TABLE 111 pickled condition, both on coated and uncoated specimens. Forthe coated specimens, a pure aluminum coating was applied by hot-dippingusing a Lundin flux, details of which are disclosed in US. Pat. Nos.2,686,354 and 2,686,355. Coating weight was about one-half ounce persquare foot of sheet (total coating weight on both surfaces). Forpurposes of comparison, oxidation tests were also run on plain carbonsteel coated with pure aluminum and with aluminum alloy containing up to10% silicon, uncoated Armco Type 409 stainless steel, and an uncoatedcommercial alloy containing 5% chromium, 0.5% molybdenum, 0.06% carbon,0.35% silicon, 0.4% manganese, residual aluminum and nickel, and balancesubstantially iron. The initial tests comprised 100 hours in still airat l,600 F, and 1,700 F, respectively. These tests are reported in TableIV below.

Since still air tests are not necessarily definitive, further specimensof coated and uncoated materials were subjected to cyclic testing, usinga cycle of 25 minutes in and 5 minutes out of the furnace for a total ofAs Welded Properties Sample 180 Bend Test Olsen Cup Test Code Parallelto Weld Heightlnches Root in Tension Face in Tension .12" diam. Flat.12" diam. Flat Root in Tension Face in Tension 15 P,F P,F P,P P,P .310,.290 .340, .360 41* P,P P,P P,F F,F .300, .300 .300, .305 42* P,P P,PP,P P,F .225, .400 .350, .340 43* P,P P,P P,P P,F .370, .370 .350, .39061* PP P.P P.P P,P .405, .390 .380, .400 62* P,P P,P P,P P,P .380, .315.370, .310 P,P P,P .065 86* P,P P,P .360

P Pass F Fail Steels of the Invention Duplicate tests in each conditionOxidation resistance tests were conducted on the samples of Table I inthe cold rolled, annealed and 130-135 cycles. These results are reportedin Table V below.

TABLE IV Oxidation Tests Hours Still Air TABLE IVContinued OxidationTests 100 Hours Still Air 1600 F 1700" F Sample Weight Weight IncreaseIncrease Code mg/in rng/in 42 uncoated 6.7 1 1.3 43 uncoated 4.1 7.8 61uncoated 12.8 14.2 62 uncoated 6.3 19.3 85 uncoated 18.1 25.0 86uncoated 2.1 3.9 Al coated 30.2 41 Al coated 12.5 11.1 42 Al coated 16.210.0 43 Al coated 13.6 10.0 61 Al coated 14.8 13.6 62 Al coated 16.6 9.886 Al coated 6.1 6.2

TABLE V superior to other samples in the coated condition despite thefact that it was subjected tocyclic test temper- Oxidation Tests atures100 F. higher than any of the other materials Sample Cyclic 25,5 w Inchtested. In the uncoated condition it was at least equiva- CodeConditions ease m /in lent to other uncoated samples.

Type 409 stainless 135 cycles 1500F 118 15 uncoated 135 cycles 1500"F666 41 uncoated 135 cycles 1500F 315 42 uncoated] 135 cycles 1500F 27043 uncoated 135 cycles 1500F 250' 61 uncoated 135 cycles 1500F 382 62uncoated 135 cycles 1500F 376 85 uncoated 132 cycles 1500F 198 86uncoated 132 cycles 1500F l 17 15 Al coated 135 cycles 1500F 37.2 41 Alcoated 135 cycles 1500F 17.4 42 Al coated 136 cycles 1500F 15.0 43 Alcoated 135 cycles 1500F 12.3 61 A1 coated 135 cycles 1500F 14.3 62'Alcoated 135 cycles 1500F 18.2 86 A1 coated 130 cycles 1600F 10.2

The oxidation tests indicate that all the steels of the presentinvention exhibited good scaling resistance both in still air and cyclictests without coatings. With aluminum coatings, the steels of theinvention are superior to Armco Type 409 in uncoated condition. Itshould further be noted that the still air tests on aluminum andaluminum alloy coated plain carbon steel base metal or substrate showedthis material to be completely unacceptable for oxidation resistance atelevated temperatures of the order of l,500 l,700 F, because ofblistering and war-page.

Optimum oxidation resistance is achieved in a steel of the inventioncontaining about 2% chromium, about 2% aluminum, about 1% silicon andabout 0.5% titanium (with titanium about 8 times the carbon content).Sample Code 86, having this approximate analysis, was

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An alloy having good oxidation resistance at elevated temperature,good weldability and good formability, consisting essentially of, byweight percent, from about 0.04 to about 0.06% carbon, from about 1.7 toabout 2.1% chromium, from about 1.7 to about 2.0% aluminum, from about0.6 to about 0.9% silicon, from'about 0.2 to about 0.4% manganese, fromabout 0.1 to about 0.6% titanium, with the titanium content being atleast about 8 times the carbon content, and remainder iron except forincidental impurities.

2. The alloy claimed in claim 1, including a maximum of about 0.05%m0lybdenum, a maximum of about 0.05% vanadium, and less than about 0.2%copper.

3. An alloy'having' good oxidation resistance at elevated temperature,good weldability and formability, consisting essentially of, by weightpercent, from about 0.04 to about 0.06% carbon, from about 1.7 to about2.1% chromium, from about 1.7 to about 2.0% aluminum, from about 0.6 toabout 0.9% silicon, from about 0.2 to about 0.4% manganese, from about0.1 to about 1.5% of an element chosen from the group consisting ofcolumbium, zirconium, and mixtures thereof, with the content of saidelement beingat least about 10 times the carbon content, and remainderiron except for incidental impurities

1. AN ALLOY HAVING GOOD OXIDATION RESISTANCE AT ELEVATED TEMPERATURE,GOOD WELDABILITY AND GOOD FORMALITY, CONSISTING ESSENTIALLY OF, BYWEIGHT PERCENT, FROM ABOUT 0.04 TO ABOUT 0.06% CARBON, FROM ABOUT 1.7 TOABOUT 2.1% CHROMIUM FROM ABOUT 1.7 TO ABOUT 2.0% ALUMINUM, FROM ABOUT0.6 TO ABOUT 0.9% SILICON, FROM ABOUT 0.2 TO ABOUT 0.4% MANGANESE, FROMABOUT 0.1 TO ABOUT 0.6% TITANIUM, WITH THE TITANIUM CONTENT BEING ATLEAST ABOUT 8 TIMES THE CARBON CONTANT, AND REMAINDER IRON EXCEPT FORINCIDENTAL IMPURITIES.
 2. The alloy claimed in claim 1, including amaximum of about 0.05% molybdenum, a maximum of about 0.05% vanadium,and less than about 0.2% copper.
 3. An alloy having good oxidationresistance at elevated temperature, good weldability and formability,consisting essentially of, by weight percent, from about 0.04 to about0.06% carbon, from about 1.7 to about 2.1% chromium, from about 1.7 toabout 2.0% aluminum, from about 0.6 to about 0.9% silicon, from about0.2 to about 0.4% manganese, from about 0.1 to about 1.5% of an elementchosen from the group consisting of columbium, zirconium, and mixturesthereof, with the content of said element being at least about 10 timesthe carbon content, and remainder iron except for incidental impurities.